The phenomenon of capacitance has been known for centuries. The earliest electrical storage device, the Leyden jar, was a simple capacitor.
In its simplest conceptual form, a capacitor 10 as shown in FIG. 1 may be considered as two conductive plates 12a and 12b set parallel with an electrically nonconductive space 14 between them. If S is the area 16 of each plate and d is the distance 18 between them, and if d is much less than the square root of S, then the capacitance value C of capacitor 10 is given byC=∈S/d where C is in units of farads, equal to coulombs stored per volt applied, and ∈ is the dielectric constant of whatever material fills space 14. For empty space (or approximately for most gases) the dielectric constant is ∈0=8.849×10−12 farad per meter. For other materials ∈ is conveniently expressed as the product of ∈0 times ∈R, where ∈R is a dimensionless number characteristic of the material. ∈R typically ranges from about 2 for perfluorinated hydrocarbons such as Teflon®, through values of 3 to 6 typical of most plastics and minerals, 8.8 for aluminum oxide, 30 for tantalum oxide and 80 for distilled water, up to about 1200 for specially processed forms of barium titanate.
Electrical communication between a capacitor 10 and the outside world is made through two conductors 20a and 20b, connected to plates 12a and 12b respectively as illustrated in FIG. 1a. As a result, in electronic diagrams a capacitor is represented by symbol 22 in FIG. 1b representing the two plates, the space between them and the conductors leading out. Almost invariably, a practical capacitor contains in addition to these an outer insulating jacket or coating to prevent unwanted current flow or leakage, as indicated by dashed line 24 of FIG. 1a. 
As is readily seen from the equation just given, capacitance may be increased either by increasing S, by increasing e, or by decreasing d. Early capacitors using empty space (or more practically, air) between their plates, as indicated by 30 in FIG. 2a, had very small values of C, on the order of a few picofarads (10−12 farad) up to a few hundred picofarads. While useful at very high frequencies or at voltages up to many thousand volts, such capacitors find little use in other modern low-voltage electronics.
The next generation of capacitors, as illustrated in FIG. 2b, placed the conductive plates much closer together, separated by a thin (typically around 10−4 meter) film 32 of mica, waxed paper or plastic. Typically the plates were formed of metal foil and the film and foil were rolled together for compactness. The resulting combined increase in S, decrease in d and increase in ∈ made practical values two to three orders of magnitude higher, from about a nanofarad (10−9 farad) to a few hundred nanofarads, though at reduced working voltages of usually a few hundred volts.
The development of ferroelectric ceramics, most of them based on specially processed barium titanate made possible a further generation of capacitors termed monolithic. Such a “chip” capacitor is built up from alternating thin (about 10−4 meter) layers of ceramic 34 and a metal such as palladium, fired together into a single ceramic piece, as illustrated in FIG. 2c. While practical devices are small, they are easier to manufacture than film capacitors and much more durable. The very high value of ∈R in ceramic 34, typically around 1200, offsets the necessarily reduced S yielding practical values from a few nanofarads to about one microfarad (10−6 farad) with working voltages typically of ten to thirty volts. Ferroelectric ceramics may also be used in “disk” capacitors where the ceramic replaces material 32 in the previous example, yielding smaller values of capacitance but able to operate at higher voltages.
A further development, permitting large capacitance in small volume through large S and very small d, was the electrolytic capacitor as illustrated in FIG. 2d. Here at least one plate is formed of a metal such as aluminum or tantalum, etched or otherwise processed to maximize its surface area, then placed in contact with an electrolyte 36. By application of a controlled current and voltage, a thin (10−6 meter or so) layer of oxide 38 is grown on the surface and forms the dielectric. Since ∈R is moderately high (though not as high as in a ferroelectric ceramic) and d is orders of magnitude thinner than would be possible in a mechanically built-up structure, capacitance values from one microfarad (10−6 farad) to many thousand microfarads are easily achieved at working voltages ranging from a few volts to a few hundred volts.
The development of electrolytic capacitors revolutionized electronics by making relatively large capacitance values economically achievable. Electrolytics have a downside, however, in that they are inherently unidirectional or polarized. This results from the manufacturing process and the chemistry of the metal and electrolyte. So long as the applied voltage has the same polarity of that used to form the oxide layer originally, the capacitor functions as intended. If the voltage is reversed, however, the oxide layer breaks down and the capacitor becomes extremely leaky resulting essentially in a short circuit. This requires special precautions in the use of electrolytic capacitors, limiting their usefulness to applications such as power storage in which the applied voltage is always of the same, correct polarity.
It is worth pointing out that processing both plates of an electrolytic capacitor 40, rather than just one, as illustrated in FIG. 2e, can yield a device with approximately symmetrical characteristics. Such symmetrical electrolytic capacitors are occasionally used in signal processing, for example in low-impedance audio applications such as filters and graphic equalizers. Because of the greater complexity of manufacture and resulting high relative cost, however, symmetrical electrolytic capacitors 40 are often a last resort when nothing else will serve. A more common approach is simply to place two conventional, polarized electrolytic capacitors of equal value back-to-back in series. Leakage through each capacitor in its reverse direction quickly charges their common point to a high enough voltage to prevent further reverse biasing, and the pair then functions approximately as would a single, symmetrical capacitor having one-half the rated value of each component.
The newest development in capacitors, and forming part of the invention described below, is the so-called “ultracapacitor,” “supercapacitor,” “double-layer” or “electrochemical” (“EC”) capacitor. This is broadly similar to an electrolytic capacitor illustrated in FIG. 2e, but instead of a layer of metal oxide, it depends for its “dielectric” on the surface barrier potential which arises at any junction between a semiconductor such as carbon and another material. As illustrated in FIG. 2f when the second material is an electrolyte 36, this barrier arises through the spontaneous formation of a so-called “double layer” in which one layer is formed by mobile electrons in the carbon 42 and the other by mobile ions in the electrolyte 36. Voltage applied across this double layer draws the opposite charges apart, leaving a thin empty zone which forms the dielectric.
Because this layer is very thin, often less than a nanometer (10−9 meter), and because the surface area S of a body of activated carbon or carbon aerogel is extremely high, practical capacitance values in devices of this sort range from a minimum of about 0.05 farad (50,000 microfarads) upward to many farads. The downside arises from this same thinness: a single capacitor of this type is limited, depending on its construction and the liquid forming the dielectric, to a maximum working voltage of no more than two or three volts. For higher voltages, multiple units must be connected in series.
In early double-layer capacitors only one body of porous carbon 42 was used, one wire connected to it through a metal plate backing the carbon, and the other wire to a metal can lined with specially processed nickel 44 in direct contact with the electrolyte. Such a device, as illustrated in FIG. 2f, has properties like those of an electrolytic capacitor (illustrated in FIG. 2e): operating in only one polarity, and liable to damage or destruction if voltage is applied the wrong way. Its nickel content 44 also makes it relatively costly and requires special handling in disposal due to possible toxic release.
Since about the turn of the twenty-first century, however, advances in technology have made it simpler and more economical to attach carbon to both plates of a symmetrical double-layer capacitor 46 rather than just one. Such a capacitor is illustrated in FIG. 2g. The resulting capacitors are suitable for mass production and prices are rapidly dropping. Working voltage per cell is typically two to three volts, with higher voltages attainable by connecting two or more cells in a series stack 48 as illustrated in FIG. 2h. Small units suitable for printed circuit board mounting are now commercially available at prices as low as about $1.00 each in quantity. One such series of devices is the ELNA “Dynacap” DX series, comprising 0.047, 0.1, 0.22, 0.33 and 1.0-farad devices all rated at 5.5 volts.
It is not generally recognized, however, that the new, symmetrical characteristics of these carbon-carbon double-layer capacitors 48 permit uses far beyond those of energy storage. A likely cause is the fact that the previous generations of high-valued capacitors, such as electrolytic capacitors, were almost exclusively unidirectional and easily damaged by reversed polarity. Since the electrical double layer is made possible with the carbon-electrolyte junction, it is not subject to such damage. If it is in any way disrupted, it can re-form virtually instantly. In a modern symmetrical double-layer capacitor 48, having two plates coated with porous carbon, one plate becomes active and provides high capacitance in one polarity while the other acts essentially as a short circuit, while in the opposite polarity their roles are reversed.
For example, FIG. 3 illustrates the self-discharge curves measured for a typical ELNA DX-5R5V473 “Dynacap” (0.047 farad, 5.5 volts) capacitor 48 in both the marked “forward” and marked “reverse” directions. In each case, the capacitor 48 was charged from a 9-volt alkaline radio battery through a 100-ohm resistor until a connected voltmeter read 8.50 volts, well above the rated Dynacap maximum working voltage of 5.5 volts, indicated by line 60 in FIG. 3a. The battery was then disconnected. The capacitor voltage was measured at increasing intervals and plotted against the logarithm of time. Smooth curves 62 and 64 were then fitted to the nominal “forward” and “reverse” data points respectively as illustrated in FIG. 3a. 
From the change in voltage between successive data points the internal leakage current was then found from the relationshipIIkg=CΔV/Δt=0.047 ΔV/Δt and plotted logarithmically as a function of voltage. As seen in FIG. 3b, the leakage drops from relatively high values near point 70 (above the rated voltage line 60) to about forty microamperes at the rated voltage, then reaches a plateau (near point 72) at about twenty microamperes over the range of four to five volts. Below four volts, the leakage drops again to lower values (around point 74). There is a difference between marked “forward” leakage values 76 and marked “reverse” values 78, but this difference is never more than a factor of two across the rated working voltage range.
One typical application for capacitors is capacitive coupling. Specifically, capacitive coupling is the transfer of electrical energy from one circuit element to another circuit element using the capacitance between the circuit elements. Capacitive coupling is typically achieved by placing a capacitor in series with the signal to be coupled. Such a capacitor may be called a coupling capacitor. A coupling capacitor is used to connect two circuits such that only the alternating current (AC) signal from the first circuit can pass through to the next while direct current (DC) is blocked. This technique may be used to avoid altering the DC bias settings of each circuit when they are interconnected. Thus capacitive coupling is also known as AC coupling.
A coupling capacitor may be known as a DC blocking capacitor. Capacitive coupling has the disadvantage of degrading the low frequency performance of a system containing capacitively coupled units, since each coupling capacitor along with the input impedance of the next stage forms a high-pass filter and each successive filter results in a cumulative filter. Thus, for adequate low frequency response, the coupling capacitor usually must have high enough capacitance so that the reactance (at the lowest frequency of interest) is much higher than the input impedance of the next stage. Poor low-frequency performance of a coupling capacitor can complicate the transfer of A/C electrical signal having long time constants.
Prior art devices, for example the bioelectronic stimulators described In U.S. Pat. Nos. 5,217,009, 5,413,596, 6,011,994, 6,321,119, 6,535,767 7,117,034, and U.S. Published Application No. 20040267333, all of which are here incorporated by reference, have required the use of back-to-back electrolytic capacitors for output direct current blocking. With capacitors of practical size, however, successful coupling is limited to signals having small unbalanced charge content at any given time. Those with significant unbalance even for a relatively short period may become distorted.
As such, there is a need in the art for capacitive coupling circuits with very good low frequency performance. Such low frequency performance can sustain efficient signal propagation where the signals may have long time constant components. There is also a need for such capacitive coupling to substantially block the transfer of direct current signal components while maintaining acceptable low frequency performance. Also, there is a need in some cases, especially in medical applications, for such a capacitive coupling circuit to contain series redundant elements for safety considerations.